Multiple myeloma (MM) is the second most common hematopoietic cancer. MM ultimately becomes refractory to treatment when self-renewing MM cells begin unrestrained proliferation by unknown mechanisms. Our goal is to develop novel and superior therapeutic strategies for MM by defining the mechanism of cell cycle dysregulation and self-renewal in MM. Cell cycle re-entry and progression through G1 is modulated by the balance between cyclins and cyclin-dependent kinases (Cdks) on one hand, and Cdk inhibitors on the other. We have demonstrated previously that inhibition of Cdk6 by p18INK4c, one of its physiologic inhibitors, is required for G1 cell cycle arrest during normal plasma cell differentiation. We have further shown that one, but not more than one, of the three D cyclins is elevated in primary bone marrow (BM) myeloma cells of each patient in vivo. Coordinated elevation and specific pairing of Cdk4/6 with D cyclins predisposes MM cells to proliferation. Moreover, dysregulation of Cdk4/6 is associated with an aggressive clinical course, suggesting that Cdk4 and Cdk6 are promising therapeutic targets. We have now identified a potent and specific small molecule Cdk4/6 inhibitor, which rapidly induces G1 arrest in primary myeloma cells ex vivo even in the presence of BM stromal cells, and prevents the growth of disseminated human myeloma tumors in xenografts. Inhibition of Cdk4/6 by PD 0332991 does not induce apoptosis. However, it markedly enhances the killing of myeloma cells by Bortesomib (Velcade, PS-341), a proteasome inhibitor widely used in MM therapy. These findings provide a strong rationale for developing cell cycle- based combination therapy for MM. On this basis, we propose that dysregulation of Cdk4 and Cdk6 in self- renewing myeloma cells by BM proliferation signals is a critical event in the initiation, progression and drug resistance in MM. To test this hypothesis and to advance superior therapeutic interventions, we propose the following specific aims: 1. To investigate the control of MM cell expansion and tumor growth by PD 0332991 in combination with proteasome inhibitors and to develop an effective cell-cycle based therapy, in primary BM myeloma cells and representative MM cell lines ex vivo and in two complementary in vivo systems;2. To characterize the role of ectodomain shedding in the generation of putative self-renewing myeloma, control their reactivation by PD 0332991 and define their normal counterparts in plasma cell differentiation;3. To elucidate the BM signals and pathways that promote self-renewal of BM myeloma cells and drug resistance.